1. Field of the Invention
The present invention relates to liquid crystal displays. More particularly, it relates to a liquid crystal display, and to a method of fabricating that display, having improved adhesion between a passivation layer and a pixel electrode.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) includes switching devices consisting of thin film transistors, each having a gate electrode, a gate insulating film, an active layer, an ohmic contact layer, and source and drain electrodes. A liquid crystal display further includes a liquid crystal material between a lower plate, provided with pixel electrodes and the switching devices, and an upper plate provided with color filters.
An LCD is comprised of N×M pixels, where N and M are integers. Each pixel includes a thin film transistor and a pixel electrode that is coupled to the thin film transistor. N gate lines and M data lines transmit signals to the gate electrodes and to the drain electrodes of the thin film transistors. The gate lines and data lines are formed in such a manner that they cross. The pixel electrodes beneficially overlap the data lines and the gate so as to increase an aperture ratio of the LCD. At the overlap of each pixel electrode and data line is a passivation layer. That passivation layer is usually formed from an organic insulator having a small dielectric constant. This reduces a parasitic capacitance between the pixel electrodes and the data lines. Organic materials such as BCB (β-staggered-divinyl-siloxane-benzocyclobutene), acrylic organic compound, or PFCB (perfluorocyclobutane) are common organic insulators.
FIGS. 1A to 1E illustrate a simplified process of fabricating a conventional LCD. Referring now to FIG. 1A, a metal thin film, usually of aluminum (Al) or copper (Cu), is formed on a transparent substrate 11, beneficially by sputtering. The metal thin film is then patterned to form a gate electrode 13. Beneficially, the fabrication of the gate electrode is performed by wet photolithography.
Referring now to FIG. 1B, a gate insulating film 15, an active layer 17, and an ohmic contact layer 19 are sequentially formed over the gate electrode 13, beneficially by chemical vapor deposition (CVD). The gate insulating film 15 is beneficially formed from an insulation material, such as silicon oxide or silicon nitride. The active layer 17 is beneficially formed from undoped amorphous silicon or undoped polycrystalline silicon. The ohmic contact layer 19 is beneficially made from an amorphous silicon or a polycrystalline silicon that is doped with an n-type or p-type impurity at a high concentration. The ohmic contact layer 19 and the active layer 17 are then photolithographically patterned using anisotropic etching to expose the gate insulating film 15. However, as shown, the active layer 17 and the ohmic contact layer 19 are left over the gate electrode 13 and over its surrounding area.
Referring now to FIG. 1C, a metal layer is then deposited over the resulting structure, beneficially using either CVD or sputtering. In particular, the metal layer is formed over the ohmic contact layer 19 so as to make electrical contact with the ohmic contact layer 19. Beneficially, the metal layer is comprised of molybdenum (Mo), chrome (Cr), titanium (Ti) or tantalum (Ta), or of a molybdenum alloy such as MoW, MoTa or MoNb. The metal layer is then patterned by photolithography to expose the gate insulating film 15. Additionally, part of the metal layer over the gate electrode 13 and part of the ohmic contact layer 19 is removed, thereby forming a source electrode 23 and a drain electrode 21. Additionally, part of the active layer 17 is exposed. The exposed portion of the active layer 17 becomes a channel.
Referring now to FIG. 1D, an organic insulation material having a small dielectric constant, beneficially an acrylic organic compound, BCB or PFCB, or the like, is deposited on the transparent substrate 11, thereby forming a passivation layer 25. In the conventional LCD, the passivation layer 25 has a hydrophobic property. Then, the passivation layer 25 is patterned to define a contact hole 27 that exposes the drain electrode 21. The passivation layer 25 is then dry ashed in a vacuum to transform its surface to have a hydrophilic property.
Referring now to FIG. 1E, an indium tin oxide (ITO), tin oxide (TO) or indium zinc oxide (IZO) transparent conductive material is deposited on the passivation layer 25 and into the contact hole 27 such that the transparent conductive material electrically contacts the drain electrode 21. As the surface of the passivation layer 25 has a hydrophilic property, the adhesion between the passivation layer 25 and the transparent conductive material is better than it would be if the surface of the passivation layer was left with a hydrophobic property. Then, the transparent conductive material is patterned by photolithography, beneficially using an acid mixture such as HCl,(COOH)2 or HCl+HNO3 as an echant, to form a pixel electrode 29.
In the conventional method of fabricating the LCD as described above, first a passivation layer having a hydrophobic property is dry ashed in a vacuum to produce a surface having a hydrophilic property, and then a transparent conductive material is deposited. The transparent conductive material is then patterned. This procedure tends to prevent the pixel electrode from being reduced in size by etching when it is being formed.
While generally successful, the conventional method of fabricating the LCD has a problem. Since the passivation layer must be dry ashed in a vacuum, the fabrication time tends to be rather long because it takes a relatively long time to form a vacuum. Therefore, a way of providing a surface having a hydrophilic property at normal process pressure, usually atmospheric pressure, would be beneficial.